MEMS-specific%20fabrication

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MEMS-specific fabrication

• Bulk micromachining
• Surface micromachining
• Deep reactive ion etching (DRIE)
• Other materials/processes

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Bulk, Surface, DRIE

• Bulk micromachining involves removing material
  from the silicon wafer itself
• Typically wet etched
• Traditional MEMS industry
• Artistic design, inexpensive equipment
• Issues with IC compatibility
• Surface micromachining leaves the wafer
  untouched, but adds/removes additional layers
  above the wafer surface, First widely used in
  1990s
• Typically plasma etched
• IC-like design philosophy, relatively expensive
  equipment
• Different issues with IC compatibility
• Deep Reactive Ion Etch (DRIE) removes substrate
  but looks like surface micromachining!

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Bulk Micromachining

• Many liquid etchants demonstrate dramatic etch
  rate differences in different crystal directions
• lt111gt etch rate is slowest, lt100gt and lt110gt
  fastest
• Fastestslowest can be more than 4001
• KOH, EDP, TMAH most common anisotropic silicon
  etchants
• Isotropic silicon etchants
• HNA
• HF, nitric, and acetic acids
• Lots of neat features, tough to work with
• XeF2, BrF3
• gas phase, gentle
• Xactix, STS selling research production
  equipment

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KOH Etching

• Etches PR and Aluminum instantly
• Masks
• SiO2
• compressive
• SixNy
• tensile
• Parylene!
• Au?

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Crystal Planes Miller Indices

• abc in a cubic crystal is just a direction
  vector
• (abc) is any plane perpendicular to the abc
  vector
• ()/ indicate a specific plane/direction
• /ltgt indicate equivalent planes/direction
• Angles between directions can be determined by
  scalar product the angle between abc and xyz
  is given by axbycz (a,b,c)(x,y,z)cos(the
  ta)
• e.g.

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Miller indices
001
abc
c
010
a
b
100
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001
abc
(abc)
1/c
c
010
b
1/b
a
1/a
100
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001
010
100
(100)
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001
010
(111)
(110)
100
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Typical 100 wafer
Cross-section in (110) plane
The wafer flat is oriented in the 110 direction
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(111)
(111)
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Rosette
Amplified etch rate
Masking layer
Lateral undercut
Un-etched silicon
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Anisotropic Etching of Silicon
lt100gt
lt111gt
54.7
Silicon Substrate

• Anisotropic etches have direction dependent etch
  rates in crystals
• Typically the etch rates are slower
  perpendicularly to the crystalline planes with
  the highest density
• Commonly used anisotropic etches in silicon
  include Potasium Hydroxide (KOH), Tetramethyl
  Ammonium Hydroxide (TmAH), and Ethylene Diamine
  Pyrochatecol (EDP)

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Etch stops in anisotropic silicon etching

• Electrochemical etch stop
• High boron doping (1e20/cm)

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Micromachining Ink Jet Nozzles
Microtechnology group, TU Berlin
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Bulk Micromachining

• Anisotropic etching allows very precise machining
  of silicon
• Silicon also exhibit a strong piezoresistive
  effect
• These properties, combined with silicons
  exceptional mechanical characteristics, and
  well-developed manufacturing base, make silicon
  the ideal material for precision sensors
• Pressure sensors and accelerometers were the
  first to be developed

Silicon pressure sensor chip
Packaged pressure sensor
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KOH etching atomic view
STM image of a (111) face with a 10 atom step.
From Weisendanger, et al., Scanning tunnelling
microscopy study of Si(111)77 in the presence of
multiple-step edges, Europhysics Letters, 12, 57
(1990).
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Bulk micromachined cavities

• Anisotropic KOH etch (Upperleft)
• Isotropic plasma etch (upper right)
• Isotropic BrF3 etch with compressive oxide still
  showing (lower right)

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Clever KOH etching of (100)
Clockwise from above Ternez Rosengren Keller
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Surface Micromachining
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Surface micromachining material systems

• Structure/ sacrificial/
  etchant
• Polysilicon/ Silicon dioxide/ HF
• Silicon dioxide/ polysilicon/ XeF2
• Aluminum/ photoresist/ oxygen plasma
• Photoresist/ aluminum/ Al etch
• Aluminum/ SCS EDP, TMAH,
  XeF2
• Poly-SiGe poly-SiGe DI water

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Residual stress gradients
More tensile on top
More compressive on top
Just right! The bottom line anneal poly between
oxides with similar phosphorous content. 1000C
for 60 seconds is enough.
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Residual stress gradients
A bad day at MCNC (1996).
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Hinges
Deposit and pattern second sacrificial
Pattern contacts Deposit and pattern 2nd poly
Etch sacrificial
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Deep Reactive Ion Etch
BOSCH Patent
STS, Alcatel, Trion, Oxford Instruments
Unconstrained geometry 90 side walls High aspect
ratio 130 Easily masked (PR, SiO2)
Uses high density plasma to alternatively etch
silicon and deposit a etch-resistant polymer on
side walls
?
?
Process recipe depends on geometry
Polymer
Polymer deposition
Silicon etch using SF6 chemistry
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Scalloping and Footing issues of DRIE
lt100 nm silicon nanowire over gt10 micron gap
microgrid
Footing at the bottom of device layer
Milanovic et al, IEEE TED, Jan. 2001.
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Typical simple SOI-MEMS Process
oxide mask layer
Si device layer, 20 µm thick
1) Begin with a bonded SOI wafer. Grow and etch a
thin thermal oxide layer to act as a mask for the
silicon etch.
buried oxide layer
Si handle wafer
silicon
2) Etch the silicon device layer to expose the
buried oxide layer.
Thermal oxide
3) Etch the buried oxide layer in buffered HF to
release free-standing structures.
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DRIE structures

• Increased capacitance for actuation and sensing
• Low-stress structures
• single-crystal Si only structural material
• Highly stiff in vertical direction
• isolation of motion to wafer plane
• flat, robust structures

Thermal Actuator
Comb-drive Actuator
2DoF Electrostatic actuator
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SCREAM fab flow
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SCREAM
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Courtesy Connie Chang-Hasnain
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Courtesy Connie Chang-Hasnain
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Courtesy Connie Chang-Hasnain
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Sub-Micron Stereo Lithography
New Micro Stereo Lithography for Freely Movable
3D Micro Structure -Super IH Process with
Submicron Resolution- Koji Ikuta, Shoji Maruo,
and Syunsuke Kojima Department of Micro System
Engineering, school of Engineering, Nagoya
University Furocho, Chikusa-ku, Nagonya 464-01,
Japan Tel [phone], Fax [phone]
E-mail ikuta_at_mech.nagoya-u.ac.jp
Fig. 6 Schematic diagram of the super IH process
Fig. 1 Schematic diagram of IH Process
Fig. 5 Process to make movable gear and shaft
(a) conventional micro stereo lithography needs
base layer (b) new super IH process needs no base
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

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Sub-Micron Stereo Lithography
New Micro Stereo Lithography for Freely Movable
3D Micro Structure -Super IH Process with
Submicron Resolution- Koji Ikuta, Shoji Maruo,
and Syunsuke Kojima Department of Micro System
Engineering, school of Engineering, Nagoya
University Furocho, Chikusa-ku, Nagonya 464-01,
Japan Tel [phone], Fax [phone]
E-mail ikuta_at_mech.nagoya-u.ac.jp
Fig. 10 Micro gear and shaft make of solidified
polymer (b) side view of the gear of four
teeth (d) side view of the gear of eight teeth
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

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Combining Microstereolithography and Thick Resist
UV Lithography
Combining Microstereolithography and Thick
Resist UV Lithography for 3D Microfabrication A.
Bertsch, H. Lorenz and P. Renaud Swiss Federal
Institute of Technology (EPFL) DMT IMS, CH
1015 Lausanne, Switzerland Tel [phone]
Fax [phone] E-mail arnaud.bertsch_at_epfl.ch
Fig. 1 Diagram of microstereolithorgraphy
apparatus using a pattern generator.
Fig. 2 Influence of the geometry on the surface
roughness.
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

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Combining Microstereolithography and Thick Resist
UV Lithography
Combining Microstereolithography and Thick Resist
UV Lithography for 3D Microfabrication A.
Bertsch, H. Lorenz and P. Renaud Swiss Federal
Institute of Technology (EPFL) DMT IMS, CH
1015 Lausanne, Switzerland Tel [phone]
Fax [phone] E-mail arnaud.bertsch_at_epfl.ch
Fig. 5 SEM image of an object made of three
imbricated springs. This structure consists of
1000 layers of 5mm each, built along the axis
direction.
Fig. 4 WEM photograph of a micro-turbine made by
microstereolithography.
Fig. 6 Enlargement of fig. 5.
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

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Combining Microstereolithography and Thick Resist
UV Lithography
Combining Microstereolithography and Thick
Resist UV Lithography for 3D Microfabrication A.
Bertsch, H. Lorenz and P. Renaud Swiss Federal
Institute of Technology (EPFL) DMT IMS, CH
1015 Lausanne, Switzerland Tel [phone]
Fax [phone] E-mail arnaud.bertsch_at_epfl.ch
Fig. 15 Two level SU-8 structure with an added
axle.
Fig. 11 Plastic injected watch gear, total
height 1.4 mm.
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany